This invention is concerned with variable displacement pumps, which are used to power and control hydraulic systems.
In a simple hydraulic system a pump draws oil from a low-pressure reservoir and supplies it at high pressure to a consumer unit(s) such as a ram. The only losses in this system are due to leakage etc., in the pump and ram, and viscous loss in the pipes, but the ram speed is directly related to the pump speed.
As the fluid volumes demanded by the consumer unit(s) will usually be variable a common way of controlling such a system is to use a controllable bypass, which returns a proportion of the pump output to the reservoir without going through the ram. The speed of the latter can clearly be varied from zero, with the bypass fully open, to the maximum speed, with the bypass completely closed. However, this is very wasteful of energy. In a second form of control, a series valve is located in the high pressure supply, but this is just as inefficient. The valve raises pump pressure above that actually required, thereby wasting energy. At higher pressures, leakages within the pump become more significant, so they act as a bypass, to control the speed.